1. Field of the Invention
The present invention relates to a method for producing a zirconia-based sintered body excellent in mechanical properties and having practically effective oxygen ionic conductivity.
2. Description of the Related Art
Cubic zirconia stabilized with yttria has an excellent oxygen ionic conductivity at high temperatures and has been noticed as functional ceramics such as solid electrolytes for fuel cells and oxygen sensors. However, since the stabilized cubic zirconia is mechanically very brittle, it lacks reliability in practical use and besides is difficult to form into large or thin products.
On the other hand, stabilized tetragonal zirconia is a sintered body obtained by stabilizing at room temperature the tetragonal crystal phase stable at high temperatures and is excellent in strength and toughness at room temperature. This is because when the stabilized tetragonal zirconia undergoes stress resulting in cracks, phase transformation of the tetragonal crystal to monoclinic crystal is brought about by the stress field at the head of the cracks and growth of the cracks is inhibited due to volume expansion caused by the phase transformation. However, since the tetragonal crystal becomes stable at high temperatures, the transformation to monoclinic crystal occurs with difficulty and mechanical properties greatly deteriorate.
Furthermore, oxygen ionic conductivity of the stabilized tetragonal zirconia is inferior to that of the stabilized cubic zirconia. Therefore, in order that zirconia can be practically used as an oxygen ionic conductor, strength and toughness of the cubic zirconia must be enhanced. There have been studied some methods for enhancing the toughness of ceramics and one effective method is the particle-dispersion-strengthening method. This method attains the strenghening by dispersing second phase particles in a matrix and the most important is that the particles to be dispersed are finely and uniformly dispersed.
Improvement of mechanical properties by the particle-dispersion-strengthening method with addition of alumina to zirconia has been practised for tetragonal zirconia. For example, T. Shimada et al ["Advances in Ceramics", Vol. 24, Page 397 (1988)] have produced a composite by mill-mixing alumina with tetragonal zirconia in which 2-3 mol % of Y.sub.2 O.sub.3 is doped to form a solid solution. As a result, a flexural strength of about 1000 MPa and a fracture toughness (K.sub.1c) of 5-6 MPam.sup.1/2 were obtained by adding alumina in an amount of 10-20% by weight. Furthermore, K. Tsukuma et al have reported that the strength can be improved to 2400 MPa by the hot isostatic pressing (HIP) ["J. of American Ceram. Soc.", Vol. 68, C-4 (1985)]. However, these are tetragonal zirconias and hence are unsuitable as oxygen ionic conductors. With reference to cubic zirconia, too, E. J. Esper et al have reported that strengthening can be attained by mill-mixing under vibration alumina with cubic zirconia in which 7.5 mol % of Y.sub.2 O.sub.3 is doped to form a solid solution ["Advances in Ceramics", Vol. 12, Page 528 (1984)]. However, in this case, the particle size of zirconia in the sintered body is large, namely, 1-5 .mu.m and besides, dispersion of alumina is ununiform.
According to these methods, zirconia powders and alumina powders are merely mechanically mixed and sintered and the products are inferior in uniformity of dispersion of the particles in the matrix of sintered body and are also unsatisfactory as oxygen ionic conductors. Thus, the object of the present invention is to provide a method for producing a zirconia-based sintered body in which alumina particles are uniformly dispersed and which is excellent in mechanical properties and has practically effective oxygen ionic conductivity.